University of South Carolina Aiken Acidic and Basic Salts Worksheet Answer the following assignment using the information included in the file. Please include your work. Acidic and Basic Conjugate Salts Their pH in Aqueous Solution
In this lab, as a thought experiment, imagine dissolving sodium chloride (table salt) in water. As
an ionic compound (a metal ion with a nonmetal ion) it dissolves as each of its individual ions
one sodium ion and one chloride ion according to the formula NaCl. Measuring the pH of this
table salt solution with a calibrated pH meter shows a pH value very close to 7 or neutral pH.
This indicates that those two ions do not disturb the acid/base chemistry in waterin other
words they are acid/base spectator ions. In fact, salt ions that are the cations of any strong base
and the anions from any strong acid (except sulfuric acid) are acid/base spectator ions. That is
why I asked you to remember the list of six strong acids in water and the six common strong
bases in water. Doing so saves time in so many ways. This means that solid salts like KNO3,
BaBr2 or LiClO4 for instance dissolved in water are A/B spectator ions. Look at your list of strong
acids and bases and you will see the cations above associated with the strong bases on that list
and the anions above associated with the strong acids on that list.
If I told you then that a salt like sodium nitrite, NaNO2, when dissolved in water gave a pH of 8.5
to 9.0, you would attribute that change in pH not to the sodium ion but to the nitrite ion, NO2.
In fact, there are many salts that contain a spectator anion or spectator cation and some other
kind of ion. Often those other ions turn out to change the pH and can be shown to be the
conjugate acid of a weak base or the conjugate base of a weak acid. Those conjugate ions, along
with a spectator ion, when they dissolve in water behave as any weak reacting material does.
They set up an equilibrium that results in a slight excess of either H + ions in the case of a
conjugate acid or a slight excess in OH ions in the case of a conjugate base. And like any
equilibrium, once we have the balanced reaction, we can set up an ICE scheme for it and
possibly predict the equilibrium concentrations of the materials in the reaction. Because the pH
is disturbed and we have a way to measure that disturbance, a calibrated pH probe, this
experiment becomes a Situation 1 calculation where a measured concentration at equilibrium
and the ICE scheme allows us to experimentally find the equilibrium constant. We will be
obtaining the equilibrium constants for these conjugate acids and bases by measuring the pH of
the various salt solutions. Once we have those conjugate equilibrium constants, we will
calculate the Ka or Kb value for the weak acid or base that gave rise to the conjugate in the first
place and compare those to the textbook values. Remember to save space, and therefore
money, the publishers often just include the Ka or Kb values and leave it up to the reader to find
Kca or Kcb.
Turn to page 153 in your lab manual and look at the first table. We will be focusing most of our
attention here. First you will need to calculate the molar masses for all four salts. Just write
them under the formula given in each box in column 1. Column 2 is the weighed mass of each
salt used in our imagined experiment. Because the procedure calls for 0.40 grams of each, we
will imagine that is what we used. So those are the entries for that column. We are supposed to
dissolve the salt in 50.0 mL of water. We can then calculate an approximate molarity by dividing
the mass of each salt by its molar mass, this gives the number of moles of salt, and dividing that
result by 0.0500 L following the definition of molarity, moles per liter. Column 3 is made up of
the entries of molarity for each salt solution. Column 4 is the last column where experimental
data is provided. The average pH values for each solution based on past experiments are as
follows: NaF ? pH = 8.1; NaC2H3O2 ? pH = 9.1; NH4Cl ? 4.8; NaHCO3 ? 10.1. Place these
values in the appropriate box in column 4. Based strictly on the measured pH you should be
able to decide if the salt behaves as an acidic material or as a base material. Those are the
entries you need to make in column 5.
Now we get down to the nitty grittyfinding the Kca or Kcb for the selection of salts. Table 2 on
page 153 is an example ICE scheme for the salt NaF. We will use it as our example. Obtain the
balanced reaction: F(aq) + H2O(l) ? HF(aq) + OH(aq). Table 2 does not include water as it
is the solvent and in dilute solutions we usually exclude the solvent for equilibrium expressions.
We need the full algebraic solution to the ICE scheme. You should be able to see that [F]o is the
value from Table 1, column 3 and the other materials are zero concentration initially. Using the
change as minus on the LHS and plus on the RHS gives us for the equilibrium concentrations:
[F]eq = [F]o x
[HF]eq = x
[OH]eq = x
Now we use our old friend the pH Cycle, 14 pH = pOH and [OH]eq = x = 10pOH. With that in
hand, I plug the needed information into the expression and solve for Kcb; Kcb = x2/([F]o x).
You do not need to use Table 2 specifically but to do the calculations, you will need to set things
1) Fill in Table 1, Columns 1 5 as described in an earlier paragraph.
2) Fill in Column 6 by finding the Kcb value for either sodium acetate or sodium bicarbonate but
not both. Because sodium fluoride was our example above, do not include it in your choices.
3) Then everyone should find the Kca value for ammonium chloride. You should be able to find
the balanced reaction in the textbook. The only other point is to realize that NH4+ is a
conjugate acid so it produces H+ ion. We therefore do not need to find the pOH or the [OH]eq
for the solution. The pH will give us the [H+]eq concentration as 10pH.
4) Once you have calculated your two K values for the conjugates, determine the K values for
their associated weak acid or weak base using the equations Kw = Ka x Kcb or Kw = Kb x Kca.
Compare the values you got for Ka and Kb for your two salts side-by-side with the values in the
Show your work in the lab notebook. You do not need to do any questions on page 154.
Purchase answer to see full
Why Choose Us
Top quality papers
We always make sure that writers follow all your instructions precisely. You can choose your academic level: high school, college/university or professional, and we will assign a writer who has a respective degree.
Professional academic writers
We have hired a team of professional writers experienced in academic and business writing. Most of them are native speakers and PhD holders able to take care of any assignment you need help with.
If you feel that we missed something, send the order for a free revision. You will have 10 days to send the order for revision after you receive the final paper. You can either do it on your own after signing in to your personal account or by contacting our support.
All papers are always delivered on time. In case we need more time to master your paper, we may contact you regarding the deadline extension. In case you cannot provide us with more time, a 100% refund is guaranteed.
Original & confidential
We use several checkers to make sure that all papers you receive are plagiarism-free. Our editors carefully go through all in-text citations. We also promise full confidentiality in all our services.
24/7 Customer Support
Our support agents are available 24 hours a day 7 days a week and committed to providing you with the best customer experience. Get in touch whenever you need any assistance.
Try it now!
How it works?
Follow these simple steps to get your paper done
Place your order
Fill in the order form and provide all details of your assignment.
Proceed with the payment
Choose the payment system that suits you most.
Receive the final file
Once your paper is ready, we will email it to you.
No need to work on your paper at night. Sleep tight, we will cover your back. We offer all kinds of writing services.
You are welcome to choose your academic level and the type of your paper. Our academic experts will gladly help you with essays, case studies, research papers and other assignments.
Admission help & business writing
You can be positive that we will be here 24/7 to help you get accepted to the Master’s program at the TOP-universities or help you get a well-paid position.
Editing your paper
Our academic writers and editors will help you submit a well-structured and organized paper just on time. We will ensure that your final paper is of the highest quality and absolutely free of mistakes.
Revising your paper
Our academic writers and editors will help you with unlimited number of revisions in case you need any customization of your academic papers